Add 'processing delay' and 'totalDelay' to the metrics reported by the Spark Streaming subsystem

As the Spark Streaming tuning guide indicates, the key indicators of a healthy streaming job are:

• Processing Time
• Total Delay

The Spark UI page for the Streaming job [1] shows these two indicators but the metrics source for Spark Streaming (StreamingSource.scala) [2] does not.

Adding these metrics will allow external monitoring systems that consume the Spark metrics interface to track these two critical pieces of information on a streaming job performance.

[1] https://github.com/apache/spark/blob/master/streaming/src/main/scala/org/apache/spark/streaming/ui/StreamingPage.scala#L127

[2] https://github.com/apache/spark/blob/master/streaming/src/main/scala/org/apache/spark/streaming/StreamingSource.scala